Re: Who - "invisible" objects
"There is no requirement in law for a pedestrian (or a fallen tree, or a concrete block, a landslide or a cow) to be wearing something which you might consider a convenience to you."
True, there is no requirement in law, but you are going against guidance in the 'Rules for pedestrians' section of the Department for Transport Highway Code if you are not making an effort to make yourself visible whilst walking beside a public road. So it's not just a case of it being for vehicle users' convenience:-
Help other road users to see you. Wear or carry something light-coloured, bright or fluorescent in poor daylight conditions. When it is dark, use reflective materials (eg armbands, sashes, waistcoats, jackets, footwear), which can be seen by drivers using headlights up to three times as far away as non-reflective materials.